In the semiconductor industry, there has recently been a high-level of activity using strained Si-based heterostructures to achieve high mobility structures for complementary metal oxide semiconductor (CMOS) applications. In such heterostructures, the strained Si is typically formed atop a relaxed SiGe alloy layer.
Relaxation of SiGe alloy layers can occur when the thickness of the layer exceeds a certain value (called the critical thickness) for a given Ge concentration in the alloy. The relaxation of strained SiGe alloy layers that are thicker than the critical thickness occurs primarily through the formation of strain-relieving misfit dislocations. Strained SiGe layers that are grown thicker than the critical thickness, but remain strained and defect-free, are said to be metastable. In fact, any strained layer whose total film energy (including the strain, thickness and defect components) is not minimized with respect to defect production is, by definition, metastable.
Metastable-strained SiGe layers can be defect free if the growth conditions are chosen correctly. Specifically, relaxation by defect formation almost always occurs at local microscopic defect sites at the strained Si/SiGe interface. The growth of metastable SiGe layers, then, is most successful when the growth surface is atomically clean and free of existing defects. Once a metastable SiGe layer is grown, it can relax if the layers are annealed at a high enough temperature. The nucleation and growth rate of misfit dislocations are strongly temperature dependent. Relaxation occurs by defect production and growth until 1) there is not enough strain energy within the film to create another dislocation, and 2) the existing dislocations stall, become pinned, or get trapped by some other mechanism.
The above physical properties of metastable strained SiGe layers put limitations on what initial SiGe layers can be applied to the thermal mixing method of fabricating SiGe-on-insulator (SGOI) substrate materials. The thermal mixing method is disclosed, for example, in co-pending and co-assigned U.S. patent application Ser. No. 10/055,138, filed Jan. 23, 2002, entitled “Method to Create High-Quality SiGe-On-Insulator for Strained Si CMOS Applications”.
If a thermodynamically stable SiGe layer is grown on a silicon-on-insulator (SOI) substrate and subsequently oxidized at high temperatures (on the order of about 1200° C. or greater), the final SGOI material formed will generally remain fully strained. This is because the only mechanism available to relieve strain at the substrate level is through defects; and since there is not enough strain energy to form defects, no relaxation occurs. If a metastable SiGe layer is grown on an SOI substrate and oxidized at high temperatures, the layer will tend towards a minimum film energy condition with respect to the residual SiGe film strain and the extent of lattice relaxation (by defect generation). In some applications, it is advantageous to form SGOI that remain fully strained, i.e., no relaxation, rather than a relaxed SiGe layer.
In view of the above, it would seem that the fabrication of fully strained SiGe-on-insulators is only possible by using thermodynamically stable SiGe layers. Such a method however places constraints on the total SGOI film thickness for a given Ge concentration.
Despite the current state of the art, applicants are unaware of any ongoing effort to create a “frustrated” SGOI film in which the SiGe layer is metastable, yet very resistant to relaxation.